Sea urchins are globular, spiny sea creatures that make up the class Echinoidea. There are around 950 species of sea urchin, and they inhabit the seabed of every ocean and every depth zone, from the tide line to 15,000 feet deep. The name urchin is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs.
The scientific name of these marine invertebrates is Echinoidia, which is also the name of their class. They can range from 3 to 10 cm (1 to 4 in) in diameter and have hard shells (tests). These tests are covered in spikes, which they are most known for, and help to protect them from predators as they cannot swim. They can move slowly along the sea floor, using small, adhesive tube feet among their spines.
Sea urchins eat algae mostly, although will also eat slow moving animals. Their main predators are sea otters and starfish, as well as the wolf eel and triggerfish. Sea urchins are edible, although not all species are, and are caught and sold in some parts of the world. They can sting humans when touched, but these stings are mostly harmless although may trigger some allergic reactions.
Most species of sea urchin are not considered to be in danger or threatened. There are some species which are, however, and this is mostly due to commercial fishing and climate change.
Sea Urchin Characteristics
Sea urchins are relatively small animals that typically range from 3 to 10 cm (1 to 4 in) in diameter, although the largest species of sea urchin, the red sea urchin, can reach a diameter of more than 18 cm (7.1 in)!
Sea urchins have a spherical body, with five-fold symmetry, that is completely covered by sharp spines. The spines can be long and sharp in some species, but shorter and wider in others. Some species, such as sand dollars, look like they don’t have any spines, but actually have short hairs all over their bodies.
These spines grow on a hard shell called the “test”, consisting of numerous calcium carbonate plates, which encloses the animal. These sea creatures also have hundreds of small, adhesive tube feet, that cover their body with their spines, which are used in movement and food transport.
The underside of a sea urchin is referred to as the “oral surface”, because it contains the mouth. The mouth of most urchins is comprised of a complex five-part, triangular, calcium-carbonate structure with large teeth and a fleshy tongue-like sucking part. The hard, triangular teeth are primarily used to scrape algae from substrate on the ocean floor.
Some sea urchins do not have the classic spherical shape — such as the sand dollars. These sea urchins are oval in shape, with distinct front and rear ends, giving them bilateral symmetry. The upper half of their body is domed, while the underside is flat and they lack the feet other sea urchins have. This is what gives them the ability to burrow into the sand.
With almost a thousand sea urchin species, they come in all different shapes, sizes and colors. The most common colors are black, white, red, orange, green, brown, purple, pink, yellow, blue and gray.
Sea Urchin Lifespan
The lifespan of sea urchins can vary greatly depending on the species. These animals could live anywhere from 15 to 200 years!
Sea Urchin Diet
Sea urchins are primarily herbivorous and the majority of their diet is made up of algae and kelp. However, they can also feed on sea cucumbers, mussels, worms, sponges, and brittle stars.
With their feeding and grazing habits, the sea urchin can help to keep ecosystems in check. However, if populations of sea urchins get too large in certain areas, they can completely strip marine environments of plant material.
Sea Urchin Behavior
Sea urchins are nocturnal, hiding in crevices during the day and coming out at night to feed. They are very sensitive to touch, chemicals and light, despite the fact that they don’t have eyes. Sea urchins are fairly social animals who prefer to live close to each other.
Sea urchins produce underwater sounds that contribute to the underwater soundscape. Sound levels in coastal waters tend to increase immediately before sunrise and after sunset, which may be an important source of information for other marine lifestyle.
Sea urchins do have the ability to bite or harm humans. Sea urchins possess venom — some carry venom in their spines, while others have venom in their tubular feet. However, most species are actually not poisonous when they sting or bite humans.
When stung by their spines or bitten by their pedicellaria, the venom will only leave puncture wounds or may trigger some allergic reactions. One species, the flower urchin, is said to be the most dangerous of all sea urchins. It has the ability to temporarily paralyze humans, and some stings by these sea urchins have even resulted in death.
Sea Urchin Reproduction
Sea urchin reproduction mostly occurs during the summer season. Sea urchins have separate male and female sexes, although no distinguishing features are visible externally.
During the breeding period, the female releases millions of tiny eggs coated in jelly into the water. Some species, however, do not let their eggs float freely in the water. Instead, these species protect their eggs by holding on to them using their spines.
Similarly, the males release sperms into the water which meet with the eggs, and fertilization occurs. A swimming embryo forms within half a day which later develops into a corn-shaped larva. For some species, these larvae come equipped with their food sources, in the form of yolk, thus eliminating the need for feeding. For most sea urchin species, though, the larva has twelve arm-like structures covered with little hairs (cilia) used for feeding, which can be used to capture food.
It can take several months for the larva to transform into a developed sea urchin. Once the larva is ready to develop fully, it sinks to the bottom of the ocean. After sinking, it takes around one hour for them to develop into adults. They usually develop into an adult at around five years of age, although some will reach sexual maturity sooner.
Sea Urchin Location and Habitat
Sea urchins live in oceans throughout the world and can found on the bottom of the sea bed, from the tide line to 15,000 feet deep. They reside where there is a lot of food, amongst coral, substrate and kelp beds. Sea urchins can be found in all climates, from warm seas to polar oceans, but most species are found on temperate and tropical coasts.
Sea Urchin Conservation Status
Sea urchins are generally abundant across the world and most species are not under threat of extinction. However, in a lot of places, sea urchins are overfished and the high demand for their meat in places like Japan could be a cause for concern. The European edible sea urchin is listed as Near Threatened on the IUCN red list because of its high demand. Collector urchins are also exploited.
In the Mediterranean, the purple sea urchin population is currently also in a near threatened state. This is not just due to fishing — factors that have decimated the species include warming sea temperatures and invasive fish that eat algae, depriving the urchins of a diet staple.
Despite this, the majority of sea urchin species are thriving, and, with 950 species of them, it is unlikely they are going to die out any time soon.
Sea Urchin Predators
The main predators of the sea urchin are shellfish like crabs and lobsters, sea otters, starfish, wolf eel and triggerfish. Fortunately, their sharp spikes can help deter predators from eating them, although predation can help to keep sea urchin populations in check.
Sea Urchin As Food
There are 18 edible species of sea urchin, including the European edible sea urchin, violet sea urchin and collector urchin. Sea urchins are not eaten whole, but the gonads of both male and female sea urchins, usually called sea urchin roe or corals, are delicacies in many parts of the world, especially Japan, where they are sold as sushi or sashimi. The green, red, and purple urchins have the highest demand by fisheries globally because their lobes tend to be larger and visually more appetizing.
Sea Urchin FAQs
Where do sea urchins live?
Sea urchins live all over the world. They reside at the bottom of the ocean amongst substrate, kelp forests and corals, which can be very close to the surface, or as deep down as 15,000 feet where it is very dark. Some prefer warmer water, while other thrive in colder climates.
Do sea urchins have brains or eyes?
Sea urchins do not have brains. Instead their whole entire nervous system acts like a brain! They also do not have eyes, but it has been indicated that they can see with their tubular feet!
Are sea urchins dangerous?
Some species of sea urchin carry venom, with urchins in tropical environments are more likely to harm. If you step on a sea urchin, you are likely to feel a sharp pain, which may be followed by side effects of nausea, vomiting, difficulty breathing and muscle weakness.
Despite this, sea urchin stings are very rarely deadly, and there have only been a few reported deaths as a result of sea urchin stings. These have mostly been divers who have drowned following muscle weakness after a sting.
Can you eat sea urchins?
Yes! There are 18 species of edible sea urchin that are harvested around the world. The edible parts of the urchin are the gonads. They are a very popular dish in Japan.
What do sea urchins eat?
Sea urchins mostly eat kelp and algae. However, some species also eat sea cucumbers, mussels and sponges. In fact, these animals will feed on almost anything that floats by!
Sea Urchin Taxonomy
Sea urchins are members of the phylum Echinodermata, which also includes other echinoderms like sea stars, sea cucumbers, brittle stars, and crinoids. Just like other members of Echinodermata, sea urchins have five-fold symmetry and move with their tiny tube feet.
Sea urchins belong to the class Echinoidea. Within this class, there are two subclasses: Euechinoidea (“modern” sea urchins, including irregular ones) and Cidaroidea or “slate-pencil urchins”, which have very thick, blunt spines, with algae and sponges growing on them. Irregular echinoids, such as sand dollars, sea biscuits, and heart urchins, are classed inside Euechinoidea.
Inside these two subclasses, there are four superorders and thirteen orders. There are around 950 species of sea urchin which are split into these orders. Take a look at the breakdown of Echinoidea below.
Subclass Perischoechinoidea
- Order Cidaroida (pencil urchins)
Subclass Euechinoidea
- Superorder Atelostomata
- Order Cassiduloida
- Order Spatangoida (heart urchins)
- Superorder Diadematacea
- Order Diadematoida
- Order Echinothurioida
- Order Pedinoida
- Superorder Echinacea
- Order Arbacioida
- Order Echinoida
- Order Phymosomatoida
- Order Salenioida
- Order Temnopleuroida
- Superorder Gnathostomata
- Order Clypeasteroida (sand dollars)
- Order Holectypoida
Sea Urchin Species
With almost 950 sea urchin species in the world, it would be impossible to list of all them here. However, we can take a closer look at some of the most common types of sea urchins, and see which genera and family they belong to.
Green sea urchin
- Superorder: Echinacea
- Order: Echinoida
- Family: Strongylocentrotidae
- Genus: Strongylocentrotus
- Species: Strongylocentrotus droebachiensis
The green sea urchin is commonly found in northern waters all around the world including both the Pacific and Atlantic Oceans. It resides on rocky substratum in the intertidal and up to depths of 1,150 meters (3,770 ft).
This urchin is in the shape of a slightly flattened globe and is named after its striking green color. This is a relatively fast growing sea urchin, and its age is generally calculable based on its size: one year for every 10 mm.
The green urchin is edible and is harvested for both locals and for export in. It has been used in fine dining by chefs in the past.
Pacific purple sea urchin
- Superorder: Echinacea
- Order: Echinoida
- Family: Strongylocentrotidae
- Genus: Strongylocentrotus
- Species: Strongylocentrotus purpuratus
The Pacific purple sea urchin is found along the eastern edge of the Pacific Ocean extending from Ensenada, Mexico, to British Columbia, Canada. It lives in lower inter-tidal and nearshore sub-tidal communities. As its name suggests, it is deep purple in color. This sea urchin can grow to about 10 cm (4 inches) and can live for 70 years.
Red sea urchin
- Superorder: Echinacea
- Order: Echinoida
- Family: Strongylocentrotidae
- Genus: Mesocentrotus
- Species: Mesocentrotus franciscanus
The red sea urchin is found in the northeastern Pacific Ocean from Alaska to Baja California. It lives in shallow waters from the low-tide line to greater than 280 m (920 ft) deep, and is typically found on rocky shores sheltered from extreme waves in areas where kelp is available. They live for between 7 to 10 years.
This sea urchin varies in color from red to dark burgundy and usually have a diameter of around 18 cm with a spine length of 50 to 75 mm. In fact, they are the largest of all the sea urchins and, in British Columbia, they grow to a diameter of over 180 cm (7 in) with 8 cm (3 in) long spines.
Red sea urchins are harvested commercially for their reproductive organs primarily for export to Japan. In California, the industry is regulated by the California Department of Fish and Game for number of licenses, harvesting time, and size.
Purple sea urchin
- Superorder: Echinacea
- Order: Camarodonta
- Family: Parechinidae
- Genus: Paracentrotus
- Species: Paracentrotus lividus
The purple sea urchin is found in the Mediterranean Sea and eastern Atlantic Ocean from western Scotland and Ireland to the Azores, Canary Islands and Morocco. It resides just below low water mark at depths down to twenty metres and sometimes also in rock pools.
This sea urchin is has a circular, flattened greenish test with a diameter of up to seven centimetres. The spines are are usually purple but are occasionally other colors including dark brown, light brown and olive green.
The purple sea urchin has been harvested for export over Croatia, Portugal and Ireland. They are considered a gonads are considered a delicacy in Lebanon, France, Italy, Spain, Malta, and parts of Croatia, most notably on the island of Korčula.
European edible sea urchin
- Superorder: Echinacea
- Order: Camarodonta
- Family: Echinidae
- Genus: Echinus
- Species: Echinus esculentus
The European edible sea urchin, also known as the common sea urchin, is found in coastal areas of western Europe, such as Portugal, Spain, France, Belgium, the Netherlands, Denmark, Norway, Sweden, the United Kingdom and Ireland. It resides in areas with hard substrates, down to a depth of 1,200 m (3,900 ft).
This sea urchin is reddish or purplish with white tubercles and grows to about ten centimetres in diameter. The spines are blunt ended and usually white with purplish tips.
As its name suggests, the European edible sea urchin is eaten as food. The species name esculentus means “edible”. Despite this, esculentus is not among the preferred sea urchin species for consumption, due to its white gonads. Sea urchin species with orange gonads are preferred.
It is listed as Near Threatened on the IUCN Red List.
Lime urchin
- Superorder: Diadematacea
- Order: Diadematoida
- Family: Diadematidae
- Genus: Diadema
- Species: Diadema antillarum
The lime urchin, also known as the black sea urchin or the long-spined sea urchin is found on the coral reefs of the western Atlantic and Caribbean basin. It resides at depths of between 1 to 10 metres.
In some tropical areas, this sea urchin one of the most abundant, widespread, and ecologically-important sea urchins. This is because it consumes algae that can otherwise grow to such an extent that it can smother coral reefs.
This sea urchin is characterized by its exceptionally long black spines, which can grow to 10 to 12 cm in length, or as long as 30 cm in very large individuals. This is a stark contrast from most sea urchin spices that have spines between 1to 3 cm in length.
Common heart urchin
- Superorder: Atelostomata
- Order: Spatangoida
- Family: Loveniidae
- Genus: Echinocardium
- Species: Echinocardium cordatum
The common heart urchin, also known as the sea potato, has cosmopolitan distribution and is found in sub-tidal regions in the North Eastern Atlantic. It resides at depths of down to 230 metres and lives buried in the sandy sea floor.
This sea urchin gets its name from its appearance. It is heart-shaped and covered in a dense mat of furrowed yellowish spines which grow from tubercles and mostly point backwards. The sea potato’s test reaches from six to nine centimetres in length.
Collector urchin
- Superorder: Echinacea
- Order: Camarodonta
- Family: Toxopneustidae
- Genus: Tripneustes
- Species: Tripneustes gratilla
The collector urchin is found the Indo-Pacific, Hawaii, the Red Sea, and The Bahamas, at depths of 2 to 30 metres (7 to 100 ft). They are named the “collector urchin” because debris tends to “collect” on them.
These sea urchins are dark in color, usually bluish-purple with white spines, although some individuals have orange spines. This color disappears when the individual dies or is taken out of the ocean, and is difficult to preserve. These sea urchins reach 10 to 15 centimetres (4 to 6 in) in size.
Collector urchins are economically important in some parts of the world. They are edible and sometimes exploited by humans, and, as a result, they have become less abundant.
Slate pencil urchin
- Superorder: Echinacea
- Order: Camarodonta
- Family: Echinometridae
- Genus: Heterocentrotus
- Species: Heterocentrotus mamillatus
The slate pencil urchin, also known as the red slate pencil urchin, or red pencil urchin, is found throughout the tropical waters of the Indo-Pacific region (from the east coast of Africa to the Pacific archipelagos), but is especially abundant in Hawaii. It resides in reefs in depths from eight to 25 meters and burrows into hard sediments like limestone, coral, and basalt.
This sea urchin gets its name from its spines, which are thicker than most sea urchin spines and resemble a pencil. These spines are rounded to triangular in cross-section and taper towards the tip. The thickness of them allow the urchin to bore into hard substrates and defend itself against predators. The spines can grow as long as 10 cm and have a white ring at their stem and alternating light and dark rings.
The slate pencil urchin is a large sea urchin, with some specimens reaching over 8 cm in diameter. Most of these sea urchins are bright red, but brown and purple colorations are also seen, and the spines are often a different color.
Black sea urchin
- Superorder: Echinacea
- Order: Arbacioida
- Family: Arbaciidae
- Genus: Arbacia
- Species: Arbacia lixula
The black sea urchin is found on the coast of the Mediterranean Sea and Macaronesian Islands (Azores, Madeira, Canary Islands), and less commonly, on the Atlantic coast of Western Africa and the Brazilian coast. It resides in shallow waters, at depths from 0 to 30 m, in rocky shores.
These sea urchins are medium-sized and characterized by their deep black color and its hemispherical shape. Its spines are roughly the same size and it has secondary spines.
The black sea urchin is often confused with the purple sea urchin (Paracentrotus lividus), although the purple sea urchin is never really black.
White sea urchin
- Superorder: Echinacea
- Order: Camarodonta
- Family: Toxopneustidae
- Genus: Tripneustes
- Species: Tripneustes depressus
The white sea urchin, also known as the sea egg, is found in the tropical eastern Pacific Ocean, in Mexico, on the western coast of Central America, in Panama, in Ecuador and around the Galápagos Islands.
This sea urchin is the largest sea urchin species in the Galápagos Islands with a mean diameter of 11.5 cm (4.5 in). The white sea urchin has a dark black, dark purple or reddish brown color with white spines. It is very similar to the West Indian sea egg (Tripneustes ventricosus) and the collector urchin (Tripneustes gratilla).
Sputnik urchin
- Superorder: —
- Order: Cidaroida
- Family: Cidaridae
- Genus: Phyllacanthus
- Species: Phyllacanthus imperialis
The Sputnik urchin, also known as the imperial lance urchin, imperial sea urchin, imperial urchin, pencil sea urchin, lance urchin, knobby sputnik sea urchin, mine urchin, and land mine sea urchin, is found in the Indo-Pacific region.
It has a black to brown test and spikes vary in color, although are mostly white. The spikes are very distinctive, being thick and blunt. This urchin grows to around 10 cm.
Kina
- Superorder: Echinacea
- Order: Camarodonta
- Family: Echinometridae
- Genus: Evechinus
- Species: Evechinus chloroticus
Kina is found all around New Zealand in shallow waters around 12 to 14 metres deep. In the north of New Zealand it is found mostly on rocky seafloor areas but also in areas of sandy seafloor. These sea urchins can reach a maximum diameter of 16 to 17 cm. Its spikes are brown, and its test turns from brown to green after it loses its spines.
Helmet urchin
- Superorder: Echinacea
- Order: Camarodonta
- Family: Echinometridae
- Genus: Colobocentrotus
- Species: Colobocentrotus atratus
The helmet urchin, also known as the shingle urchin is found on wave-swept intertidal shores in the Indo-West Pacific, particularly on the shores of Hawaii. It is deep maroon color and can grow to the size of a softball.
This sea urchin has a very unique appearance, with the upper surface a mosaic of tiny polygonal plates formed from modified spines to form a smooth mosaic. This upper surface is fringed by a ring of large, flattened modified spines. On the underside, there is another ring of smaller flattened spines.
The unique shape of the helmet urchin makes them three times as resistant as other species to being washed away by moving water.
Burrowing urchin
- Superorder: Echinacea
- Order: Camarodonta
- Family: Echinometridae
- Genus: Echinometra
- Species: Echinometra mathaei
The burrowing urchin is found on reefs in tropical parts of the Indo-Pacific Ocean, from Madagascar, the East African coast and the Red Sea to Hawaii. It resides at depths down to 139 metres (456 ft).
This sea urchin grows to a diameter of around 5 centimetres (2.0 in). The color of this urchin can vary, but the test is usually dark and the spines are either green and purple with purple tips or entirely green with purple tips.
Its name comes from its ability to dig itself into the basaltic and calcareous rock where it lives. Because of its burrowing activities, it causes bioerosion of coral reefs.
Flower urchin
- Superorder: Echinacea
- Order: Camarodonta
- Family: Toxopneustidae
- Genus: Toxopneustes
- Species: Toxopneustes pileolus
The flower urchin are widespread and common in the tropical Indo-West Pacific. They reside among coral reefs, coral rubble, rocks, sand, and seagrass beds at depths of 0 to 90 m (0 to 295 ft).
These sea urchins are relatively large sea urchins and can reach a maximum diameter of around 15 to 20 cm (6 to 8 in). Their test has five segments separated from each other by five other segments. The test varies in color, usually a deep red and grey, though there are rare instances of green and pale purple.
The most striking feature of the flower urchin, and the reason for its name, is its pedicellariae (stalked grasping appendages). Flower urchins possess four types of pedicellariae, one of which, the globiferous pedicellariae, resembles flowers. These help to defend the sea urchin from predators and are pinkish-white to yellowish-white in color with a central purple dot and a bright white rim.
The flower urchin is considered highly dangerous, as it is capable of delivering extremely painful and medically significant stings when touched.
Violet sea urchin
- Superorder: Echinacea
- Order: Temnopleuroida
- Family: Toxopneustidae
- Genus: Sphaerechinus
- Species: Sphaerechinus granularis
The violet sea urchin is found in the Mediterranean Sea and the eastern Atlantic Ocean, from the Channel Islands south to Cape Verde and the Gulf of Guinea. It lives on rocks covered with seaweed or gravelly substrates.
This sea urchin is a large sea urchin that can grow up to 15 centimeters in diameter. Its test is flattened and it has spines that are short and blunt, all the same length, and arranged neatly in rows. There are two distinct color forms; the test is purple in both but one has purple spines and the other white.
The gonads of the violet sea urchin are considered a delicacy in Italy, Provence and Catalonia.
Fire urchin
- Superorder: Diadematacea
- Order: Diadematoida
- Family: Diadematidae
- Genus: Astropyga
- Species: Astropyga radiata
The fire urchin, also known as the red urchin, false fire urchin or blue-spotted urchin, is found in the tropical Indo-Pacific region, at a maximum depth of about 70 metres (230 ft) but more normally at 10 to 30 metres (33 to 98 ft). It lives in sand, shingle or coral rubble substrate.
It has a test diameter of up to 20 cm (8 in) and the spines are up to 4 cm (1.6 in) long. V-shaped areas on this urchin are red with lines of iridescent blue dots while the color of the rest of the test and spines varies from reddish brown to purple, dark brown or nearly black.
These urchin’s quills are quite venomous although not deadly to humans. Thanks to its bright color, this urchin is visible to divers.
Common sand dollar
- Superorder: Gnathostomata
- Order: Clypeasteroida
- Family: Echinarachniidae
- Genus: Echinarachnius
- Species: Echinarachnius parma
The common sand dollar is found in the North Pacific and Northwest Atlantic, on the North American east coast from New Jersey north, as well as in Alaska, Siberia, British Columbia, and Japan. It inhabits isolated areas on sandy bottoms below the low tide level down to a depth of 5,000 feet (1,500 m).
Sand dollars are species of flat, burrowing sea urchins. They are closely related to sea cucumbers and starfish and have a hard outer shell, which is covered in very fine hairs.
The common sand dollar is round, flat and disc-like, typically measuring 3 inches (7.6 cm) in diameter. Their color is a purplish brown, becoming bleached white when washed ashore. They are often taken from beaches as souvenirs.
There are three subspecies of the common sand dollar.
Banded sea urchin
- Superorder: Diadematacea
- Order: Diadematoida
- Family: Diadematidae
- Genus: Echinothrix
- Species: Echinothrix calamaris
The banded sea urchin, also known as the double spined urchin is found throughout the tropical waters of the Indo-Pacific region, from eastern coast of Africa to French Polynesia, including Hawaii and the Red Sea. It resides between the surface and a depth of 70 meters (230 ft) and and can be found in lagoons, external reef slopes and channels.
This sea urchin has a slightly oval test that reaches a diameter of 5 cm. It has two different sets of spines; shorter and slender closed spines which go from yellow to dark in color and can deliver a nasty sting, and longer and thicker spines that are often banded with light and dark color, that can reach 10 to 15 cm in length.
A dark form of this sea urchin is reported in Indian Ocean, which is often not banded and has a green sheen and a more or less reddish test.
More Fascinating Animals to Learn About
Sea Snakes
Galapagos Sea Lions
Galapagos Green Sea Turtle
Sea Lion
Stellers Sea Cow
Bigfin Squid
